Various processes are currently known in the state of the art, for the production of urea.
In particular, the synthesis of urea is effected by reacting ammonia and carbon dioxide at a high pressure and temperature, followed by separation of the urea from the mixture containing the non-reacted products and recycling of the same to the reactor.
All industrial processes for the preparation of urea are therefore based on direct synthesis according to the following reaction:2NH3+CO2CO(NH2)2+H2O  (1)
This synthesis is carried out in two different reaction steps:NH3+CO2(NH2)COONH4  (1a)(NH2)COONH4CO(NH2)2+H2O  (1b)
In the first step (1a), an exothermic equilibrium reaction takes place, having a high reaction rate at room temperature, which, however, at the high temperatures necessary for step (1b), requires high pressures in order to reach a favourable equilibrium.
In the second step (1b), an endothermic reaction takes place, which only reaches a significant rate at high temperatures (>150° C.) with a state of equilibrium which at 185° C., starting from a mixture of reagents in a stoichiometric ratio, leads to a CO2 conversion of slightly over 50%. This unsatisfactory conversion can be conveniently increased by raising the NH3/CO2 ratio.
The above-mentioned two reaction steps do not normally take place in separate areas of the reactor, but contemporaneously in the reaction mixture, said mixture therefore comprising urea, water, ammonia, carbon dioxide and ammonium carbamate, with a relative concentration, in different areas of the reactor, depending on the various thermodynamic and kinetic factors that contribute to the process.
Processes for obtaining urea by direct synthesis starting from ammonia and carbon dioxide have been widely described in specific literature in the field. An extensive review of the most common processes for the production of urea can be found, for example, in the publication “Encyclopaedia of Chemical Technology” Ed. Kirk-Othmer, Wiley Interscience, third ed. (1983), vol. 23, pages 548-575.
Industrial processes for the production of urea, normally carry out the synthesis in a reactor fed with NH3, CO2 and aqueous solutions of ammonium carbonate and/or carbamate coming from the recycled streams of non-converted reagents, at temperatures ranging from 150 to 215° C., at pressures of at least 13.2 MPa (130 atm), with a NH3/CO2 molar ratio ranging from 2.5 to 5, calculated with respect to the sum of the feeding streams, including ammonia and CO2 in the form of ammonium carbamate/carbonate. In addition to the water formed and excess of NH3 fed, the reactor effluent still has considerable amounts of CO2, mainly in the form of non-converted ammonium carbamate.
A further essential aspect for obtaining an optimal conversion is also the control of the thermal level in the reactor, as both excessively high and also excessively low temperatures lead to a reduction in the conversion due to the competition of various chemical and thermodynamic factors.
The separation of urea from the water and non-converted reagents is effected in several sections operating at decreasing temperatures and pressures, in which the decomposition of ammonium carbamate to NH3 and CO2 is effected, which are then available for recycling to the reactor. The section for the separation and recycling of the carbamate has investment costs which heavily influence the cost of the final product.
Known processes which operate according to the above general scheme are, for example, described in U.S. Pat. No. 4,092,358; U.S. Pat. No. 4,208,347; U.S. Pat. No. 4,801,745 and U.S. Pat. No. 4,354,040.
In particular, the urea contained in the aqueous solution leaving the reactor is separated from most of the non-transformed ammonium carbamate and excess ammonia used in the synthesis, in a suitable decomposer-evaporator (hereinafter called “stripper”, which is the term normally used in the field) operating at pressures the same or slightly lower than the synthesis pressure.
The decomposition of the ammonium carbamate is effected in the stripper providing heat from the outside using indirect thermal exchange with a warmer fluid, normally vapour at 1.8-3.0 MPa, possibly stripping the decomposition products with inert gases or ammonia or carbon dioxide or mixtures of inert gases with ammonia and/or carbon dioxide, the stripping possibly also being effected by exploiting the excess ammonia dissolved in the urea solution (self-stripping), and consequently without having to feed the stripping agent separately.
The decomposition products of the carbamate, together with the possible stripping agents, with the exception of the inert products, are normally condensed in condensers, obtaining a liquid which is recycled to the synthesis reactors.
Further documents that can be mentioned for reference purposes are U.S. Pat. No. 4,314,077; Great Britain Patent No. 1,184,004; Great Britain Patent No. 1,292,515; U.S. Pat. No. 3,984,469; U.S. Pat. No. 4,137,262; German Patent No. 2,116,267 and French Patent No. 2,489323, all describing processes for the production of urea with the above-mentioned characteristics.
Particularly delicate steps in the synthesis process of urea are those in which the ammonium carbamate is present at the highest concentration and temperature, and consequently in the processes mentioned above, these steps coincide with the decomposition-stripping and condensation steps of ammonium carbamate.
One of the problems to be solved in these steps is the corrosion of the equipment used, caused by the extremely aggressive characteristics that take place inside the same, due to both the presence of a high concentration of saline solutions and also as a result of mechanical stress phenomena of the surfaces in the decomposition and release areas of the gaseous phase.
In order to overcome certain of these drawbacks, certain of the known art suggests, for example, the use of special materials in producing the stripper, in particular Ti, Zr, special urea-grade stainless steels, or combinations of the same. Again according to the state of the art, it is advantageous to feed a certain quantity of air or other passivating agent, in order to prolong the corrosion resistance of the materials, especially stainless steels, favouring the formation of a stable layer of oxide on the surfaces exposed to contact with the process fluids.
At present, in this type of plant, in order to effect the passivation of the stripper (especially if the surfaces exposed to corrosion are made of titanium or stainless steel) a certain quantity of air is added at the bottom of the stripper. This addition is effected using a specific injection of air using compressors explicitly prepared for this purpose. In the other parts of the high-pressure urea synthesis loop which require passivation, this passivation is effected, on the contrary, again with air, which is mixed during the suction phase of the CO2 compressor and is sent through the compressor to the urea reactor. The air which has not participated in the passivation reaction in the reactor, leaves the reactor together with the reaction mixture and is sent to the upper part of the stripper, then passing to the carbamate condenser and from here to the carbamate separator, thus leaving the synthesis loop through the valve destined for the pressure control of the loop itself, normally also used for purging the inert products.
During this way, the air effects the passivation of the surfaces of the equipment the air encounters, which would otherwise be subjected to corrosive processes.
In consideration of what is specified above (i.e., the fact that the passivation air is sent from the reactor to the upper part of the stripper), the bottom of the stripper is excluded from the passivation action exerted by said air, which is mixed during the suction phase of the CO2 compressor and sent to the reactor through the compressor.
For this reason, certain of the known art describes the necessity of effecting a specific injection of air at the stripper bottom using compressors explicitly prepared for this purpose.
This solution, however, requires further dedicated devices (i.e., compressors), which, in addition to having a cost, also require periodic maintenance interventions.
Alternative procedures have been proposed for the feeding of a passivating agent (particularly air or oxygen at low concentrations), tending to avoid the use of further high-pressure pumping mechanisms, such as, for example, the scheme described in PCT Patent Application No. WO08/141832, in which a part of fresh carbon dioxide containing passivation air is fed, after compression, to the bottom of the stripper, where it exerts a passivating action on the surfaces most exposed to corrosion.
Although this expedient avoids resorting to the separate pumping of the passivation air, this expedient however requires a careful control of the process conditions in the synthesis cycle with self-stripping based on NH3, due to the reduced amount of CO2 sent to the reactor.